Some advice, if I may. Don't use an individual alert for each and every incorrect entry. Set a variable called warn to "", and for each incorrect entry:
warn += "New message for correcting.\n";
Then at the end of the function, if warn == "", return true, else alert(warn) and return false.
Also, at the end of the function you currently have return true spelled as return ture, and it immediately follows the return false with no closing } between them.
Another thing - web documents should not have spaces in the name.
<link href="Main Sheet.css" rel="stylesheet" type="text/css" />
Change the filename to Main_Sheet.css, or Main-Sheet.css, or MainSheet.css, and then correct the href accordingly.